1. Field of the Invention
This invention relates, in general, to electrical measuring apparatus and, more specifically, to electrical apparatus for measuring the resistance of an inductor.
2. Description of the Prior Art
Since the DC resistance of a metallic conductor varies according to the temperature of the conductor, knowledge about the DC resistance provides an indication of the temperature of the conductor. In electrical apparatus, such as power transformers, knowledge about the temperature of the inductors or windings is often desirable during the design and testing of a new winding structure. Since accessibility to the interior of these windings is often inconvenient, the measurement of the DC resistance of the winding furnishes a convenient and reliable method for determining the temperature within the winding.
Conventional methods for measuring the DC resistance of large inductors use Kelvin or Wheatstone bridge procedures. However, using these methods is unsatisfactory for certain inductors due to the long time constant associated with inductors having a large inductance and a low resistance. In large power transformers, time constants greater than one hour have been encountered. Since the temperature measurements are usually desired at a particular time or after a certain period of operating time, delays in measuring the temperature related resistance detracts from the accuracy of the information desired when conventional methods are used. In addition, the actual time needed to make the measurements is undesirable since excessive waiting is involved for the measurements to stabilize.
In order to obtain a DC resistance measurement on a large inductor accurately, the time constant of the inductor should not influence the resistance readings. Therefore, it is desirable, and it is an object of this invention, to provide apparatus for measuring the DC resistance of large inductors, wherein the apparatus contains time constant compensating circuitry in order that the readings may be taken quickly and without the need to wait until the transient voltages in the inductor have decayed sufficiently.